The invention relates to glass ceramic compounds that can store energy of X-rays and that can be (photo)-stimulated to release the stored energy. The invention encompasses also a method for recording and reproducing X-ray images using a glass ceramic compound.
A well known use of phosphors is in the production of X-ray images. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted image-wise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
According to another method of recording and reproducing an X-ray pattern disclosed e.g. in U.S. Pat. No. 3,859,527 a special type of phosphor is used, known as a photo-stimulable phosphor, which being incorporated in a panel is exposed to incident pattern-wise modulated X-rays and as a result thereof temporarily stores therein energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are can be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called xe2x80x9cdigital radiographyxe2x80x9d or xe2x80x9ccomputed radiographyxe2x80x9d.
The image quality that is produced by a conventional as well as by a digital radiographic system, depends largely on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by vacuum deposition of phosphor material on a substrate. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when phosphor crystals with high crystal symmetry are used. Phosphor having complicated crystal structures as, e.g., alkaline earth fluorohalides, tend to decompose (partially) under vacuum deposition and the production of screens by vacuum deposition while using phosphors with complicated crystal structure is quasi impossible and leads to sub-optimal results. The use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured screens and binder-less screens.
In e.g. U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali-metal phosphor in a pile-like structure is disclosed. However the vacuum deposition of these phosphors as crystal needles is not that straightforward and remains a quite expensive way in producing storage phosphor screens.
It has been proposed to incorporate phosphor particles in solgel glass, as in e.g. WO-A-95/18196 wherein the glass acts not only as support for the phosphor, but also as a medium protecting the phosphor from environmental influences, the material however is subject to processing limitations since the material is degraded when heated. In U.S. Pat. No. 5,670,086 it has been proposed to incorporate BaBr2 and BaFBr phosphors in glasses formed from a ternary mixture of B2O3-BaO-BaBr2 or B2O3-BaO-BaFBr that are doped with e.g. Eu2+. Such a mixture forms a glass ceramic upon firing wherein the phosphors are present as crystalline material with particle sizes in the range of 1 to 25 xcexcm, preferably in the range of 2 to 10 xcexcm. The relatively large phosphor particles do induce scatter and therefore the need for storage phosphor screens combining the advantages of screens with vacuum deposited phosphor needles with good environmental stability and low scatter is still not fully met.
In EP-A-779 254 fluoroaluminate glasses capable of storing X-ray energy and of releasing this energy as visible light upon stimulation are disclosed, in some of these glasses part of the fluoride ions has been replaced by chloride ions.
PSL (Photo Stimulable Luminescence) effects in homogeneous glasses doped with scintillating ions have been disclosed. In Applied Physics Letters 71 (1) Jul. 7, 1997 p43-45, PSL is reported in Cerium doped Alkali-Borate glasses. In Journal of Non-Crystalline Solids 209 (1997) p200-203, PSL is reported in Europium-Samarium doped borate glasses. In Journal of Non-Crystalline Solids 222 (1997) p290-295 PSL is reported in Cerium doped silicate glasses. In Applied Physics Letters 71 (6) Aug. 11, 1997 p759-761 PSL is reported in Europium doped fluoroaluminate glasses.
In all examples described the references cited above the glasses are quenched in order to avoid devitrification and the recorded PSL effects are very weak and hence the samples described in the references given above are of low practical use.
It is an object of the invention to provide a means for producing storage phosphor screens wherein the advantages of screens with vacuum deposited phosphor needles can be combined with good environmental stability and low scatter.
Further objects and advantages of the present invention will become clear from the description hereinafter.
The objects of this invention are realised by providing a glass ceramic material for storing energy of X-rays and releasing said energy by photo-stimulation comprising a fluoride glass matrix characterised in that:
micro-crystalline particles are embedded in said glass matrix, and said micro-crystalline particles have an average particle size, d, so that d less than 2 xcexcm, and
said glass ceramic shows in a XRD spectrum a continuous spectrum of said glass matrix and discrete peaks superimposed on said continuous spectrum. Specific features for preferred embodiments of the invention are disclosed in the dependent claims.